Hydrogen storage alloy and method for preparation thereof

ABSTRACT

A hydrogen storage material which is an AB5 type hydrogen storage alloy having a CaCu5 type crystal structure represented by general formula:wherein Mm denotes a misch metal, 4.0&lt;a&lt;=4.3, 0.25&lt;=b&lt;=0.4, 0.25&lt;=c&lt;=0.4, 0.3&lt;=d&lt;=0.5, and 5.05&lt;=a+b+c+d&lt;=5.25,or general formula:wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0&lt;a&lt;=4.3, 0.25&lt;=b&lt;=0.4, 0.25&lt;=c&lt;=0.4, 0.3&lt;=d&lt;=0.5, 0&lt;e&lt;=0.1, and 5.05&lt;=a+b+c+d+e&lt;=5.25,characterized in that the lattice length on the c-axis is 404.9 pm to 405.8 pm.

TECHNICAL FIELD

The present invention relates to a hydrogen storage material and aprocess for producing the same. More particularly, it relates to ahydrogen storage material which is, while with a minimized cobaltcontent, excellent in insusceptibility to grain size reduction andhydrogen storage characteristics (PCT characteristics) and exhibits notonly excellent initial activity that is an important characteristic foruse in a battery but high discharge characteristics for use in electrictools or low-temperature characteristics for use in hybrid electricvehicles, and a process for producing the same.

BACKGROUND ART

Nickel-hydrogen storage batteries (secondary batteries) having ahydrogen storage material in the anode have recently been attractingattention as high capacity alkaline storage batteries taking the placeof nickel-cadmium storage batteries. The hydrogen storage materials thatare currently used widely are composed of five elements, i.e., Mm (mischmetal), Ni, Al, Mn, and Co.

Compared with La-based alloys, the Mm—Ni—Mn—Al—Co alloys enableconstructing an anode out of relatively cheap materials and provideclosed nickel-hydrogen storage batteries having a long cycle life and asuppressed inner pressure rise which is caused by gas generated in caseof an overcharge and have therefore been used widely as an electrodematerial.

The Mm—Ni—Mn—Al—Co alloys currently used are designed to have aprolonged cycle life by preventing the alloys from reducing their grainsize. It is generally known that about 10% by weight of Co (0.6 to 1.0in an atomic ratio) is required to prevent the grain size reduction ofthe alloy. It is also accepted that a given amount of Co is necessaryfor securing excellent hydrogen storage characteristics andanticorrosion.

However, the material cost increases with the Co content, which isproblematical from the aspect of material cost. Taking application ofthe hydrogen storage material to large batteries into consideration,such as the power source of electric vehicles, and the ever expandingmarket of nickel-hydrogen storage batteries, in particular, the materialcost is weighty in choosing anode materials and has been a matter ofconcern.

To settle the above problem, Japanese Patent Application Laid-Open No.213319/97 proposes altering the composition of the Mm—Ni—Mn—Al—Co alloyand adding thereto a small amount of an additional element. Use of thehydrogen storage material powder disclosed therein as an anode makes itfeasible to reduce the Co content and yet to suppress deterioration ofthe anode caused by the alloy's reduction in grain size below a certainlevel and thereby to extent the cycle life of the battery.

Because the alloy of the composition disclosed in the 213319/97 does notalways secure stability in its characteristics, the present inventorshave proposed in Japanese Patent Application Lain-Open No. 152533/99 acomposition and a production process for obtaining satisfactory initialactivity, whereby a low-Co alloy has now come to be used in specialapplications.

However, where the hydrogen storage materials disclosed in the abovepublications (Laid-Open No. 213319/97 and Laid-Open No. 152533/99 areused, the batteries have insufficient discharge characteristicsparticularly in low temperature and cannot be used for electric toolsneeding high discharge characteristics or for hybrid electric vehicles.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the present invention is to provide a hydrogenstorage material of which the production cost is reduced by extremelydecreasing its cobalt content and which exhibits excellentinsusceptibility to grain size reduction, excellent hydrogen storagecharacteristics, satisfactory discharge characteristics, andsatisfactory initial activation and a process for producing the same.

As a result of extensive studies, the present inventors have found thatthe above object is accomplished by a hydrogen storage material of AB₅structure having a specific stoichiometric composition (B site rich),particularly a composition of 4.0<Ni≦4.3 and 0.25≦Mn≦0.4, and the c-axisof which is in a given range. They have also found that such a

hydrogen storage material is obtainable with the above-describedspecific composition when a casting temperature and heat treatingconditions satisfy a given relationship.

The present invention has been reached based on the above findings andprovides a hydrogen storage material which is an AB₅ type hydrogenstorage alloy having a CaCu₅ type crystal structure represented bygeneral formula:

MmNi_(a)Mn_(b)Al_(c)Co_(d)

wherein Mm denotes a misch metal, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4,0.3≦d≦0.5, and 5.05≦a+b+c+d≦5.25, or general formula:

MmNi_(a)Mn_(b)Al_(c)Co_(d)X_(e)

wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0<a≦4.3,0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, 0<e≦0.1, and 5.05≦a+b+c+d+e≦5.25,

characterized in that the lattice length on the c-axis is 404.9 pm ormore.

The present invention also provides a preferred process for producingthe hydrogen storage material of the present invention which comprisesheat-melting raw materials of a hydrogen storage material, casting themelt, and heat treating the resulting alloy in an inert gas atmosphereto produce an AB₅ type hydrogen storage material having a CaCu₅ typecrystal structure represented by the following general formulae,characterized in that the casting temperature is 1350 to 1550° C., thepouring temperature is 1230 to 1430° C., and conditions of said heattreating are 1070 to 1100° C. and 1 to 6 hours. General formula:

MmNi_(a)Mn_(b)Al_(c)Co_(d)

wherein Mm denotes a misch metal, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4,0.3≦d≦0.5, and 5.05≦a+b+c+d≦5.25, or general formula:

MmNi_(a)Mn_(b)Al_(c)Co_(d)X_(e)

wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0<a≦4.3,0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, 0<e≦0.1, and 5.05≦a+b+c+d+e≦5.25.

The Best Mode for Carrying out the Invention:

In the above formulae, Mm donates a misch metal, a mixture of rare earthelements such as La, Ce, Pr, Nd, and Sm. The hydrogen storage materialis an AB₅, type hydrogen storage alloy having a CaCu₅ type crystalstructure having a B site-rich nonstoichiometric composition ofAB_(5.05) to AB_(5.25).

In this hydrogen storage material, the compositional ratio (atomicratio) of Ni_(a)Mn_(b)M_(c)Co_(d) fulfills the following relationships.The ratio of Ni: 4.0<a≦4.3. The ratio of Mn: 0.25≦b≦0.4. The ratio ofAl: 0.25≦c≦0.4. The ratio of Co: 0.3≦d≦0.5. (a+b+c+d) is in a range offrom 5.05 to 5.25.

The compositional ratio (atomic ratio) of Ni_(a)Mn_(b)Al_(c)Co_(d)X_(c)(wherein X is Cu and/or Fe) satisfies the following relationships. Theratio of Ni: 4.0<a≦4.3. The ratio of Mn: 0.25≦b≦0.4. The ratio of Al:0.25≦c≦0.4. The ratio of Co: 0.3≦d≦0.5. The ratio of X: 0<e≦0.1.(a+b+c+d+e) is in a range of from 5.05 to 5.25.

As described above, the ratio of Ni, a, is from 4.0 to 4.3, desirablyfrom 4.1 to 4.2. If a is less than 4.0, the discharge characteristicsare not satisfactory. If it exceeds 4.3, deterioration ininsusceptibility to grain size reduction or life characteristics isobserved.

The ratio of Mn, b, is from 0.25 to 0.4. If b is less than 0.25. theplateau pressure increases, and the hydrogen storage capacity isreduced. If it exceeds 0.4, the alloy undergoes considerable corrosionso that the battery voltage greatly decreases during storage.

The ratio of Al, c, is from 0.25 to 0.4. If c is smaller than 0.25, theplateau pressure, which is the hydrogen release pressure of a hydrogenstorage material, increases to deteriorate energy efficiency in chargesand discharges. If it exceeds 0.4, the hydrogen storage capacity isreduced.

The ratio of Co, d, is 0.3 to 0.5. If d is less than 0.3, the hydrogenstorage characteristics or the resistance to grain size reduction aredeteriorated. If it exceeds 0.5, the ratio of Co is too high to realizecost reduction.

The ratio of X, e, is from 0 up to 0.1. If e is more than 0.1, thedischarge characteristics are impaired, and the hydrogen storagecapacity is reduced. (a+b+c+d) or (a+b+c+d+e) (these sums willhereinafter be sometimes referred to as x, inclusively) is from 5.05 to5.25. If x is smaller than 5.05, the battery life or theinsusceptibility to grain size reduction is ruined. If x is greater than5.25, the hydrogen storage characteristics are reduced and, at the sametime, the discharge characteristics are also deteriorated.

The hydrogen storage material of the present invention has a latticelength on the c-axis of 404.9 pm or more, preferably 404.9 to 405.8 pm.If the lattice length on the c-axis is shorter than 404.9 pm, the alloyhas poor insusceptibility to grain size reduction and reduced initialactivation (relative magnetization). Hydrogen storage materials whosec-axis lattice length exceeds 405.8 pm are not only difficult to producebut have greatly reduced hydrogen storage capacity.

The c-axis lattice length of the hydrogen storage material has differentpreferred ranges according to the value of (a+b+c+d) or (a+b+c+d+e),i.e., the value x. The value x being 5.05 or greater and smaller than5.15, the c-axis lattice length is preferably 404.9 or greater andsmaller than 405.4 pm. The value x ranging from 5.15 to 5.25, the c-axislattice length is preferably 405.4 to 405.8 pm.

Although the lattice length on the a-axis of the hydrogen storagematerial of the present invention is not particularly limited, it isusually from 500.3 to 501.0 pm.

The process of producing the hydrogen storage material of the presentinvention is then described.

Raw materials of the hydrogen storage material are weighed to give thealloying composition described above and mixed up. The mixture is meltedinto a melt by means of a high frequency induction furnace based oninduction heating. The melt is poured into a casting mold, for example,a mold of water cooling type at a casting temperature of 1350 to 1550°C. to obtain a hydrogen storage material. The pouring temperature is1200 to 1450° C. The term “casting temperature” as used herein means thetemperature of the melt in the crucible at the beginning of casting, andthe term “pouring temperature” means the temperature of the melt at theinlet of the casting mold (i.e., the temperature of the melt beforeentering the casting mold).

The resulting hydrogen storage material is heat treated in an inert gasatmosphere, for example, in argon gas under heat treating conditions of1070 to 1100° C. and 1 to 6 hours. The cast alloy structure usuallyshows fine grain boundary segregation chiefly of Mn. The heat treatmentis to level the segregation by heating.

There is thus obtained a hydrogen storage material which has a reducedcobalt content and yet exhibits excellent insusceptibility to grain sizereduction, excellent hydrogen storage characteristics, satisfactorydischarge characteristics, and satisfactory initial activation.

The hydrogen storage material, after crushed and pulverized, is suitablyused as an anode of high-discharge alkaline storage batteries. Thealkaline storage batteries thus provided are satisfactory in initialactivation and low-temperature high-rate characteristics, and the anodeof which is prevented from deterioration due to the alloy getting finerand therefore secures a long cycle life.

The present invention will further be illustrated in the concrete by wayof Examples and the like.

EXAMPLES 1-1 to 1-4, COMPARATIVE EXAMPLES 1-1 to 1-2, AND REFERENCEEXAMPLES 1-1 to 1-3

Raw materials of a hydrogen storage material were weighed to make analloying composition of MmNi_(4.13)Mn_(0.35)Al_(0.32)Co_(0.4) (AB5.2)and mixed up. The mixture was put in a crucible, and the crucible wasset in a high frequency melting furnace. After evacuating to a degree ofvacuum of 10⁻⁴ to 10⁻⁵ Torr, the mixture was heat melted in an argon gasatmosphere and cast into a copper casting mold of water cooling type at1350° C. (pouring temperature: 1250° C.) to obtain an alloy. Theresulting alloy was heat treated in an argon atmosphere under theconditions shown in Table 1 to obtain a hydrogen storage material.Reference Example 1-1 shows the characteristics of a conventional alloycontaining 10 wt % of Co, and Reference Examples 1-2 and 1-3 show thecharacteristics of conventional alloys containing 5 wt % of Co.

EXAMPLES 2-1 TO 2-3 AND COMPARATIVE EXAMPLES 2-1 TO 2-2

Hydrogen storage materials were obtained in the same manner as inExample 1-2, except for changing the pouring temperature as shown inTable 2.

EXAMPLES 3-1 TO 3-4 AND COMPARATIVE EXAMPLES 3-1 TO 3-2

Hydrogen storage materials were obtained in the same manner as inExample 1-2, except for changing the stoichiometric ratio as shown inTable 3.

EXAMPLES 4-1 TO 4-4 AND COMPARATIVE EXAMPLES 4-1 to 4-2

Hydrogen storage materials were obtained in the same manner as inExample 1-2, except for changing the alloy composition toMmNi_(4.13)Mn_(0.35 -y)Al0.32Co0.4X_(y) (AB_(5.2)) (X: Fe or Cu),wherein y was varied as shown in Table 4.

Evaluation of Characteristics:

The PCT capacity, the relative magnetization, and the grain sizeretention of the hydrogen storage materials obtained in Examples andComparative Examples were determined in accordance with the followingmethods. Evaluation of Examples and Comparative Examples was made basedon the data of the conventional 10 wt % Co-containing hydrogen storagematerial—PCT capacity: 0.82 to 0.83; and grain size retention: 0.90 to0.91. The results obtained are shown in Tables 1 to 4.

PCT Capacity:

Calculated from the hydrogen absorption isotherm measured at 45° C. H/M:0 to 0.5 MPa.

Relative Magnetization:

The hydrogen storage material was ground to powder and surface treated.Magnetization attributed to residual Ni and Co was measured andrelatively expressed in terms of a ratio to the magnetization of theabove-described 10% Co-containing hydrogen storage material powder.

Grain Size Retention:

Hydrogen gas of 30 bar was introduced into the hydrogen storage materialhaving a grain size adjusted to 22 to 53 micrometers in a PCT apparatusand then desorbed therefrom. Hydrogen absorption and desorption wererepeated 10 times, and the ratio of the average grain size after thecycle test to that before the cycle test was obtained.

TABLE 1 Grain Relative Discharge Example & Heat Lattice Lattice PCT SizeMagnetiz- Character- Compara. Treatment Length Length Capacity Retentionation istics Example (° C.-hr) B/A (a/pm) (c/pm) (H/M) (%) (%) (mAh/g)Ref. Ex. 1-1 1060-3 5.0 499.1 405.6 0.82 92 100 215 Ref. Ex. 1-2 1060-35.2 500.9 406.3 0.82 92 82 180 Ref. Ex. 1-3 1080-3 5.2 500.9 406.4 0.8293 83 170 Compa. Ex. 1-1 1060-3 5.2 500.7 404.6 0.84 82 93 231 Ex. 1-11070-3 5.2 500.5 405.6 0.82 94 104 218 Ex. 1-2 1080-3 5.2 500.5 405.50.82 95 106 220 Ex. 1-3 1090-3 5.2 500.3 405.4 0.82 96 103 217 Ex. 1-41100-3 5.2 500.4 405.5 0.81 97 99 210 Compa. Ex. 1-2 1120-3 5.2 500.7404.4 0.83 84 85 231

TABLE 2 Grain Relative Discharge Example & Lattice Lattice PCT SizeMagnetiz- Character- Compara. Pouring Length Length Capacity Retentionation istics Example Temp. (° C.) B/A (a/pm) (c/pm) (H/M) (%) (%)(mAh/g) Comp. Ex. 2-1 1180 5.2 500.7 404.6 0.84 90 93 190 Ex. 2-1 12305.2 500.5 405.6 0.82 94 103 217 Ex. 2-2 1330 5.2 500.5 405.7 0.82 93 106219 Ex. 2-3 1430 5.2 500.3 405.5 0.82 92 102 216 Comp. Ex. 2-2 1480 5.2500.6 404.8 0.81 83 84 203

TABLE 3 Grain Relative Discharge Example & Heat Lattice Lattice PCT SizeMagnetiz- Character- Compara. Treatment Length Length Capacity Retentionation istics Example (° C.-hr) B/A (a/pm) (c/pm) (H/M) (%) (%) (mAh/g)Comp. Ex. 3-1 1080-3 5.00 501.4 404.6 0.88 83 107 240 Ex. 3-1 1080-35.05 501.2 404.9 0.86 92 103 229 Ex. 3-2 1080-3 5.10 500.8 405.1 0.85 91106 219 Ex. 3-3 1080-3 5.15 500.6 405.4 0.83 93 106 217 Ex. 3-4 1080-35.25 500.1 405.7 0.80 95 102 216 Comp. Ex. 3-2 1080-3 5.30 499.2 406.00.78 96 84 193

TABLE 4 Grain Relative Discharge Example & Heat X_(y) Lattice LatticePCT Size Magnetiz- Character- Compara. Treatment (molar Length LengthCapacity Retention ation istics Example (° C.-hr) ratio) (a/pm) (c/pm)(H/M) (%) (%) (mAh/g) Ex. 4-1 1080-3 Fe 0.05 500.4 405.6 0.81 93 102 207Ex. 4-2 1080-3 Fe 0.1 500.2 405.8 0.80 95 98 201 Comp. Ex. 4-1 1080-3 Fe0.15 500.8 406.2 0.77 97 91 173 Ex. 4-3 1080-3 Ca 0.05 500.5 405.5 0.8292 103 213 Ex. 4-4 1080-3 Cn 0.1 500.6 405.7 0.81 91 101 212 Comp. Ex.4-2 1080-3 Cu 0.15 500.7 406.0 0.78 82 84 193

As is apparent from the results in Tables 1 through 4, Examples have aPCT capacity, a grain size retention and discharge characteristics ingood balance on higher levels than Comparative Examples, substantiallyequally to the conventional 10 wt % Co-containing hydrogen storagematerial (Reference Example 1-1). It is also understood that Examplesgenerally have a higher relative magnetization than ComparativeExamples, being superior in initial activation.

Industrial Applicability:

The hydrogen storage material of the present invention has an extremelyreduced cobalt content and therefore enjoys a reduction in productioncost. It is excellent in resistance against grain size reduction andhydrogen storage characteristics and satisfactory in dischargecharacteristics and initial activation.

The production process according to the present invention provides theabove-described hydrogen storage material stably and efficiently.

What is claimed is:
 1. A hydrogen storage material which is an AB₅hydrogen storage alloy having a CaCu₅ crystal structure represented bygeneral formula: MmNi_(a)Mn_(b)Al_(c)Co_(d) wherein Mm denotes a mischmetal, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, and5.05≦a+b+c+d≦5.25, characterized in that the lattice length on thec-axis is 404.9 to 405.8 pm.
 2. The hydrogen storage material accordingto claim 1, wherein (a+b+c+d) is 5.05 or greater and smaller than 5.15,and said lattice length on the c-axis is from 404.9 to 405.4 pm.
 3. Thehydrogen storage material according to claim 1, wherein (a+b+c+d) isfrom 5.15 to 5.25, and said lattice length on the c-axis is from 405.4to 405.8 pm.
 4. A process for producing a hydrogen storage materialcomprising heat-melting raw materials of a hydrogen storage material,casting the melt, and heat treating the resulting alloy in an inert gasatmosphere to produce an AB₅ hydrogen storage material having a CaCu₅crystal structure represented by the following general formula,characterized in that the casting temperature is 1350 to 1550° C., thepouring temperature is 1200 to 1450° C., and conditions of said heattreating are 1070 to 1100° C. and 1 to 6 hours, General formula:MmNi_(a)Mn_(b)Al_(c)Co_(d) wherein Mm denotes a misch metal, 4.0<a≦4.3,0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, and 5.05≦a+b+c+d≦5.25.
 5. A hydrogenstorage material which is an AB₅ hydrogen storage alloy having a CaCu₅crystal structure represented by general formula:MmNi_(a)Mn_(b)Al_(c)Co_(d)X_(e) wherein Mm denotes a misch metal, X isCu and/or Fe, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, 0<e≦0.1, and5.05≦a+b+c+d+e≦5.25, characterized in that the lattice length on thec-axis is 404.9 to 405.8 pm.
 6. The hydrogen storage material accordingto claim 5, wherein (a+b+c+d+e) is 5.05 or greater and smaller than5.15, and said lattice length on the c-axis is from 404.9 to 405.4 pm.7. The hydrogen storage material according to claim 5, wherein(a+b+c+d+e) is from 5.15 to 5.25, and said lattice length on the c-axisis from 405.4 to 405.8 pm.
 8. A process for producing a hydrogen storagematerial comprising heat-melting raw materials of a hydrogen storagematerial, casting the melt, and heat treating the resulting alloy in aninert gas atmosphere to produce an AB₅ hydrogen storage material havinga CaCu₅ crystal structure represented by the following general formula,characterized in that the casting temperature is 1350 to 1550° C., thepouring temperature is 1200 to 1450° C., and conditions of said heattreating are 1070 to 1100° C. and 1 to 6 hours, General formula:MmNi_(a)Mn_(b)Al_(c)Co_(d)X_(e) wherein Mm denotes a misch metal, X isCu and/or Fe, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, 0<e≦0.1, and5.05≦a+b+c+d+e≦5.25.